The length of a dipole antenna is related to its operating frequency. A dipole antenna typically has two radiating elements having a common center feed point. The length of the combined dipole radiating elements is typically a multiple of the transmitting or receiving frequency. For example, the dipole radiating elements may have a length that is ¼, ½, or ¾ the wavelength of the radio frequency (RF) energy. In order to operate in two frequency bands, the antenna structure must have two sets of dipole radiating elements with two different lengths.
In certain applications, such as in an instrument landing system (ILS) of an aircraft, a dual-frequency dipole antenna is used to receive the radio frequencies of the glide slope and localizer radio frequency transmissions. In these applications, the antenna is typically mounted inside the nose cone of the aircraft where space is severely limited. Therefore, it is desirable to provide a dual-frequency dipole antenna that will fit within the confines of available space and not interfere with other equipment on board the aircraft.